A number of factors bind together to produce a shotgun pattern. First, it is important to note that pattern testing produces only a flat, two-dimensional look at the pellets in flight without regard to velocity or lethal quality. 

Because patterning requires firing a shell and the components used are destroyed and separated the pattern test is a destructive test and can never be repeated. General tendencies of a particular load recipe can be examined, but by no means, will your next round be the same as the last.

Factors that we have found to have impact upon pattern results.

(1.) Shotgun stability. (I.e. Hand held or correctly fired from a padded rest).
(2.) The type of reloading device used to provide the load and the quality and/or type of final crimp.
(3.) The shotgun barrel [chamber, forcing cones, bore diameter and choke].
(4.) Weather conditions including temperature, air density and barometric pressure.
(5.) Powder lot numbers, age and batch.
(6.) Component combination and placement of components. Was it measured on an accurate scale?
(7.) Shot size and type of pellets.
(8.) Baseline variables - not everyone is working with the same factors.

Any or all of these elements string together to skew results toward the unknown. As we "read" a pattern board and produce a fixed result percentage number, we conclude that the totality of many random errors is a fixed number in the universe, bound to repeat itself over and over. Worse yet, is the shooter who decides to lay the entire blame for a poor pattern result on just one component involved in an unfortunate pattern. The component and shotgun combinations can be guided toward better patterns by affecting the variables involved (reducing velocity, using smaller pellets, etc). However, the shooter must remember that the cure is sometimes detrimental to the overall effectiveness of a load (e.g. lowering load velocity to get a "better" pattern at long range.)

PATTERN QUALITY FACTORS
Remember, the pattern result, at best, is a two dimensional representation of a four dimensional problem. The pattern paper shows no perspective of the length of the shot string, the time it took to get there nor the lethality of the pellet that penetrated it.

RELOADING TOOLS
Your reloading tools will often establish a great deal of differentiation in your loads. Not all reloading tools are created equally and often the tool is directly associated with the quality of crimping, correct sizing, uniformity and positive results. Measured results with trap loads, where the only difference was the reloading tools used to load the shells, demonstrated a wide variation of pattern results.

PELLETS
The pellets used for patterning can greatly influence the resulting pattern. For example, it can become very difficult to review results of steel shot loads. Often quite large shot is used in steel shot loads, offering yet another variable to the pattern percentage and counts. Larger shot is always more difficult to pattern with higher percentages as the pellet count in the load is reduced - thus raising the overall percent total attributed to each pellet either in or out of the magic circle. Swings up and down in percentage are magnified and are usually regarded with disproportional significance by the shooter.

LOAD VELOCITY
The velocity of the load also affects large steel shot patterns. Steel shot when fired at higher velocities will often take on a "knuckle-ball" effect and drift wildly. The drift is in any direction, including up. The knuckleball effect introduces into steel loads a significant shot to shot variability and herein is the hunter's dilemma: Slower moving steel shot will rarely penetrate a birds tough exterior of feathers, skin, fat and muscle to get into the body core, where the bird's vital organs are located. Pellets must enter the body core in order to achieve a quick kill. On the other hand, higher speed of the steel pellets contributes to a "patchy" pattern. The hunter's dilemma is a balance between pattern and pellet velocity. With steel it is difficult to achieve both within the same load and the shooter that wants results in the field will opt towards the higher velocity loads as a load that kills is much more attractive than a piece of paper that looks like a colander.

VELOCITY VERSUS PATTERN
With high pellet count/low velocity steel loads, the hunter may be superficially wounding birds rather than producing a desired "quick kill". Many of these wounded birds die at a later time and place.  That is poor conservation and unfortunately, a misconception held by too many shooters.   When you analyze the percentages, and opt for performance, higher speed loads with reduced pellet counts will provide a higher percentage of clean kills regardless of reduced pattern percentages in a fixed area.    Patterns do not kill, pellets kill.

STEEL SHOT PATTERNS
Steel shot maintains a "memory." The harder the pellet, the more the pellet will retain a "memory" of the launch influence. We have termed this the "bell" effect. The steel pellet will vibrate and continue to react to the influences established during the firing process far more than a softer bodied pellet, which absorbs energy through deformation. The "bell" effect imparts a response unique to each pellet. Every pellet will individually react to it's own set of influences, thus reducing commonality or "pattern consistency". This means that steel shot pellets are individually influenced by any and all loading and firing components and characteristics to a degree that far exceeds the characteristics of lead shot pellets.

PATTERN SUMMARY
Patterning is only half of the story. It is important to shoot and direct the pellets with a consistent "cloud" of shot, but the cloud must be moving at lethal velocity.   The shotgunner can walk the line of maintaining quality patterns and high velocities. BALLISTIC PRODUCTS is dedicated to this effort. Our hunting and shooting is as important to us as it is to you. We also desire the finest possible hunting loads to take into the field. We are constantly working on all these ballistic questions and appreciate input from our friends, the hunters in the field.